ALet's see if I can do this one justice. When it comes down to testing cylinder heads on a flow bench, you are flowing in one direction at a constant depression against the restriction of the port. This gives you an average airflow in cubic feet per minute (cfm). When an engine is running, the intake system has very chaotic flow. Let's take a middle-of-the-road racing camshaft duration and say the valve is open 300 degrees of crankshaft rotation. Each intake cycle, you must first get the air column moving, ram the cylinder full, and come to a complete stop when the valve closes, then reverse the direction of the flow in the port and inlet manifold runner. This happens every cycle that the engine rotates. As you know, it takes 720 degrees of crankshaft rotation to complete the four-stroke cycle. Also, the piston is constantly moving down the bore on the intake stroke, at different speeds based on location in the bore, and doesn't have a constant suction (or depression) on the inlet system. With all this being said, the port is at peak flow for a very limited amount of time in this 720 degrees. This is why we're constantly looking for more airflow out of cylinder head designs. Also, you don't want to make the ports arger just for the sake of airflow. Youwant the smallest port that flows the most air. You need small (proper size for the displacement) ports to keep the velocity high. This proper-sized port will always run the best and accelerate the engine the fastest.
On street-performance engines, it's very rare to see 100 percent volumetric efficiency on the engine dyno. This is measured airflow at the carburetor. Yes, if it was average flow by the minute, we could easily fill up the cubic-inch displacement of the engine. Don't we all wish it was that easy?
Overseas Education
Q I hate to ask such a rudimentary question of a guy who writes such technically filled answers, but we've all got to start somewhere, right?
I want to educate myself on basic and advanced engine and drivetrain perfor - mance, so I've started to read your magazine every month, but I'm finding that I'm missing some fundamental knowledge that will help me understand what the writers are talking about. For example, what are the differences between a 427 and a 383 block, between H-beam and I-beam rods, dual-plane and single-plane intakes? How do you decide on a rearend? What cam and what rocker ratio to run? I realize these might be easy to most of your readers, but again, I'm just back into building cars.
I know all these answers don't exist in one book, but can you suggest a few to get me moving in the right direction? I've got lots of time on my hands now and don't want it to go to waste. I'm serving as a staff officer in Baghdad, and once I return, I'll transition into life as a civilian which will afford me much more time to continue my engine education and future buildups. I plan to find a car or truck (early '60s/late '70s) somewhere in the neighborhood of 30-50 percent complete and finish it. Thanks for your time.William Richardson CPT, ENBaghdad, Iraq
A It's great that you wish to use your downtime for absorption of Bow Tie knowledge! Some of the questions you have are far from rudimentary. Design answers are somewhat straightforward; however, the camshaft selection, gearing, and powerbands can elude the most seasoned veteran.